Photoconductive elements, also called photoreceptors, are composed of a conducting support and at least one photoconductive layer which is insulating in the dark but which becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge in such areas. The resulting charge pattern on the surface is referred to as an electrostatic latent image. The latent image can be made visible by application of a liquid or dry developer containing finely divided charged toner particles which, if desired, can be transferred and fixed to another surface such as a sheet of paper.
Numerous photoconductive materials have been described as being useful in electrophotography. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines and the like.
Photoconductive elements or photoreceptors can comprise single or multiple active layers. Those with multiple active layers (sometimes called multiactive elements or composite elements) have at least one charge-generation layer and at least one charge-transport layer. Under actinic radiation, the charge-generation layer generates mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to form the electrostatic latent image.
The majority of known photoconductors are sensitive to ultraviolet and visible electromagnetic radiation. However, increasing use is being made of diode lasers which emit radiation principally in the near-infrared region of the electromagnetic spectrum, i.e., from 700 nm to about 900 nm. Many photoconductors either have little or no sensitivity to such radiation, or they have other disadvantages. For example, they become increasingly conductive in the dark and lose their ability to hold an electrostatic charge (a process known as dark decay), or they have weak absorption of infrared radiation or poor quantum efficiency, both of which result in low electrophotographic sensitivity.
There is, therefore, a need for photoconductive elements sensitive to the near-infrared region of the electromagnetic spectrum and having low dark decay and improved sensitivity.
Borsenberger et al in U.S. Pat. No. 4,471,039 have disclosed that when the .beta.-phase of a halogenated indium phthalocyanine pigment is used as the charge-generation layer in a multiactive electrophotographic element, the element has high sensitivity in the near-infrared region.
Hung et al in U.S. Pat. No. 4,666,802 have described a novel C-centered monoclinic bromoindium phthalocyanine pigment which is useful in the charge generation layer of a photoconductive element and which is sensitive to radiation in the near-infrared region of the spectrum.
A need exists for further improvement in the sensitivity of these and other photoconductive materials to infrared radiation as emitted, for example, by Ga.sub.x Al.sub.1-x As diode lasers. A further need exists for such infrared sensitive materials which also have bimodal or bipolar sensitivity. A bipolar or bimodal electrophotographic element is an element that can discharge effectively either negative or positive surface potentials when exposed to electromagnetic radiation.